Majority of the silicone surfactants used in commercial applications are nonionic, where commonly a polyoxyalkylene provides the hydrophilicity. Cationic silicone surfactants are also known and have found utility, especially in hair care applications. By comparison, anionic silicone surfactants are not as well known, presumably due to the limited and difficult synthetic routes to such materials.
One such route to anionic silicone surfactants is based on the reaction of propane sultone with amino functional siloxanes, such as taught in J. Polymer Science, Polymer Chem. Ed., vol. 17, pp 3559-3636 (1979).
U.S. Pat. No. 4,496,705 teaches a method for producing aminoalkyl-siloxane polymers of a high molecular weight by hydrolyzing a difunctional aminoalkyl-silane prior to incorporation into the siloxane polymer. The high molecular weight siloxane polymers of the '705 patent may be converted to zwitterionic siloxane rubbers by reaction with propane sultone.
U.S. Pat. Nos. 4,525,567 and 4,523,002 teach siloxane polymers containing zwitterions on tertiary silicone atoms and their aminoalkyl siloxane polymer intermediates are provided. The aminoalkyl siloxane polymer intermediates and the zwitterionic siloxane polymers of the '567 and '002 patents are obtained by co-polymerizing trifunctional aminoalkyl silanes or zwitterionic silanes, respectively, with hydroxy-terminated siloxane oligomers in the presence of an acid catalyst. The aminoalkyl siloxane polymer intermediates are converted to zwitterionic siloxane polymer by reaction with an organosultone or an organolactone.
Industrial processes to make an anionic silicone surfactant based on using propane sultone must address the safety issues due to the toxicity of the propane sultone. Also, the sulfonate polymers synthesized from the propane sultone tend to be yellow in color. Thus, a need exists to identify processes to make anionic silicone surfactants, and in particular sulfonate silicone surfactants, using intermediates that are easier to handle than propane sultone. The present inventors have identified such a process based on reacting 2-sulfobenozoic acid cyclic anhydride, sulfobenzoic acid sodium salt or derivatives thereof with an aminofunctional siloxane. The resulting compounds, while being produced by a safer route than using propane sultone, also were found to have unexpected performance benefits in comparison to the anionic silicones prepared by the propane sultone.